Running control apparatus and running control method for vehicle

ABSTRACT

A running control apparatus for a vehicle includes a required driving output deciding unit that decides a required driving force of a driving system of the vehicle on the basis of an integrated value of a difference between a target vehicle speed and an actual vehicle speed, a driving control unit that controls the actuation of the driving system on the basis of the required driving force, and a determination unit that determines whether or not the required driving force is unrealizable in the control of the driving system due to the intervention of additional control. The required driving output deciding unit imposes a limit on an increase or a decrease in the required driving force when the determination unit determines that the required driving force is unrealizable in the control of the driving system.

INCORPORATION BY REFERENCE

This application is a continuation of U.S. application Ser. No.12/023,540, filed Jan. 31, 2008, which claims priority under 35 U.S.C.119 to Japanese Patent Application No. 2007-022790, filed Feb. 1, 2007.The disclosures of U.S. application Ser. No. 12/023,540 and JapanesePatent Application No. 2007-022790, including the specification,drawings and abstract are incorporated herein by reference in thereentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a running control apparatus and a runningcontrol method for a vehicle such as an automobile or the like. Morespecifically, the invention relates to a running control apparatus and arunning control method for a vehicle that control a driving force of thevehicle or a driving torque of a driving system (an engine, a motor, orthe like) such that a vehicle speed of the vehicle coincides with atarget vehicle speed.

2. Description of the Related Art

In recent years, vehicles such as automobiles and the like have beenmounted with “automatic running control apparatuses” for automaticallycontrolling vehicle speed through, for example, “cruise control”, “creeprunning control” (Japanese Patent Application Publication No. 2004-90679(JP-A-2004-90679)), “downhill assist control” (Japanese PatentApplication Publication No. 2006-213294 (JP-A-2006-213294)), and thelike. When such an automatic running control apparatus is actuated, atarget value of a vehicle speed (target vehicle speed) is first decidedaccording to an arbitrary method, for example, through designation orselection of the vehicle speed by a driver or on the basis of a runningstate of a vehicle (or a maneuvering state of the driver) or the like.Then, a difference between the target vehicle speed and an actual valueof the vehicle speed (actual vehicle speed) is transmitted as feedbackto a control device (driving control device) for a driving system of thevehicle such as an engine, a motor, or the like, and a driving force ora driving torque is controlled such that the actual vehicle speedcoincides with the target vehicle speed, namely, that the differencebetween the target vehicle speed and the actual vehicle speed iseliminated. According to automatic running control as described above,the driver is not required to adjust a depression amount of anaccelerator pedal or a brake pedal for controlling the vehicle speed toa certain value or holding the vehicle speed constant. Therefore, theburden imposed on the driver in maneuvering the vehicle is drasticallyreduced. The aforementioned automatic running control apparatus can alsobe used in, for example, remotely operating the running of the vehicle.

In feedback control based on the difference between the target vehiclespeed and the actual vehicle speed in the aforementioned automaticrunning control apparatus, a required driving force or a requireddriving torque is often decided using an integrated value (or anaccumulated value) (over time) of the difference between the targetvehicle speed and the actual vehicle speed to ensure good follow-upperformance of the actual vehicle speed to the target vehicle speed. Adriving force or a driving torque needed to achieve or maintain acertain vehicle speed varies depending on the running state of thevehicle. Therefore, in adjusting the driving force or the driving torqueusing a quantity proportional to the difference between the targetvehicle speed and the actual vehicle speed, it is difficult toappropriately set a proportional coefficient thereof, namely, a feedbackgain in feedback of the difference between the target vehicle speed andthe actual vehicle speed (the responsiveness of the actual vehicle speeddeteriorates when the gain is small, and the actual vehicle speed tendsto overshoot when the gain is large). On the other hand, in adjustingthe value of the driving force or the driving torque on the basis of theintegrated value of the difference between the target vehicle speed andthe actual vehicle speed, the adjustment amount of the driving force orthe driving torque can be increased as the difference between the targetvehicle speed and the actual vehicle speed increases, in comparison withthe case of making the adjustment using the quantity proportional to thedifference between the target vehicle speed and the actual vehiclespeed. Even when the feedback gain is held small to prevent theoccurrence of overshoot, the actual vehicle speed can be made to swiftlyfollow the target vehicle speed. The vehicle speed as a target value ofcontrol is a first-order lag of the driving force or the driving torquethat is changed in control. Therefore, a delay in the response of thevehicle speed for a change in the driving force or the driving torque iscompensated for by controlling the driving force or the driving torqueon the basis of the integrated value of the difference between thetarget vehicle speed and the actual vehicle speed.

Incidentally, in an actual vehicle, various types of control (interruptcontrol) for controlling vehicle speed, driving force, or driving torqueare performed in some cases during the actuation of an automatic runningcontrol apparatus as described above, interrupting the actuation or inpreference to the actuation. For example, when the rotational speed ofan engine becomes excessively high with respect to the rotational speedsof wheels and an automatic transmission is overheated while a drivingtorque is transmitted from the engine to each of the wheels via theautomatic transmission, the automatic transmission sometimes performs insome cases control for reducing the rotational speed of the engine(stall control) or the like for a control device for the engine. In suchcases, the adjustment of the driving torque for vehicle speed control bythe automatic running control apparatus is not achieved. As a result,the actual vehicle speed deviates from the target vehicle speed.

In the control configuration of the aforementioned automatic runningcontrol apparatus, if the actual vehicle speed remains deviant from thetarget vehicle speed (of the automatic running control apparatus) duringthe performance of interrupt control, which is performed in preferenceto the control of the automatic running control apparatus, theintegrated value of the difference between the target vehicle speed andthe actual vehicle speed increases. In consequence, the adjustmentamount of the driving force or the driving torque required of thedriving system through feedback control by the automatic running controlapparatus (the required driving force or the required driving torque)keeps increasing as well. In this situation, when vehicle speed controlis resumed in response to the termination of interrupt control and thereestablishment of a state where the control by the automatic runningcontrol apparatus is effectively reflected, the driving system such asthe engine or the like is actuated with the increased adjustment amountof the required driving force or the required driving torque. Thus, asudden rise or fall in an actually generated driving force or anactually generated driving torque (the actual driving force or theactual driving torque) is caused. In some cases, as a result, thebehavior of the vehicle is seriously disturbed, or a sudden rise or astagnation in vehicle speed is caused. As a matter of course, it is alsoconceivable to stop the actuation of the automatic running controlapparatus during the performance of interrupt control. In such a case,however, the response of the actual vehicle speed in returning to thetarget vehicle speed may be delayed when vehicle speed control by theautomatic running control apparatus is resumed.

In the related art, however, as described above, there seems to be noproposal to control a vehicle speed in consideration of a case whereinterrupt control has been performed during the actuation of anautomatic running control apparatus.

SUMMARY OF THE INVENTION

The invention provides an automatic running control apparatus thatserves to make an actual vehicle speed coincident with a target vehiclespeed or maintain the actual vehicle speed at the target vehicle speed,is configured on the assumption that the control performed by theautomatic running control apparatus may not be reflected due to theactuation of an additional control apparatus, and is designed to curb aninconvenience resulting from an increase in (an absolute value of) anintegrated value of a difference between a target vehicle speed and anactual vehicle speed during the actuation of the additional controlapparatus. More specifically, the invention provides an automaticrunning control apparatus so configured as to suppress a sudden rise ora sudden fall in a driving force or a driving torque that is generatedin a driving system of a vehicle upon the establishment of a state wherethe control by the automatic running control apparatus is reflectedafter interruptive actuation of an additional control apparatus.

In a first aspect of the invention, a running control apparatus for avehicle includes a vehicle speed difference calculating unit thatcalculates a difference between a target vehicle speed and an actualvehicle speed, a required driving output deciding unit that decides arequired driving force or a required driving torque of a driving systemof the vehicle on the basis of an integrated value of the differencebetween the target vehicle speed and the actual vehicle speed such thatthe actual vehicle speed coincides with the target vehicle speed, adriving control unit that controls actuation of the driving system onthe basis of the required driving force or the required driving torque,and a determination unit that determines whether or not the requireddriving force or the required driving torque is unrealizable in thecontrol of the driving system due to the intervention of additionalcontrol. In this running control apparatus, the required driving outputdeciding unit imposes a limit on an increase or a decrease in therequired driving force or the required driving torque when thedetermination unit determines that the required driving force or therequired driving torque is unrealizable in the control of the drivingsystem.

Running control to which the apparatus in the first aspect of theinvention is applied may be an arbitrary type of automatic runningcontrol for automatically making the actual vehicle speed of the vehiclecoincident with the target vehicle speed as described above, such ascruise control, creep running control, downhill assist control, or thelike. It is appropriate for the target vehicle speed to be set accordingto various methods (a) through designation or selection by a driver ofthe vehicle, (b) on the basis of a drive maneuver amount of the driverof the vehicle, (c) on the basis of a gradient of a road surface onwhich the vehicle runs, or (d) in accordance with another arbitraryrunning state of the vehicle or another arbitrary running environmentcondition of the vehicle. It is appropriate for the control forcontrolling the actuation of the driving system through interruption ofthe functioning of running control in the first aspect of the inventionto be, for example, stall control as mentioned above, idle speedcontrol, traction control, vehicle behavior stabilization control (VSC),or the like.

As described already, (the absolute value of) the integrated value ofthe difference between the target vehicle speed and the actual vehiclespeed increases when the target vehicle speed cannot be achieved due tothe intervention of additional control during the performance of runningcontrol for making the actual vehicle speed coincident with the targetvehicle speed. Thus, under normal circumstances, the absolute value ofthe required driving force or the required driving torque according torunning control is supposed to greatly deviate from the actual drivingforce or the actual driving torque that is actually generated in thevehicle. Then, upon reestablishment of a state where the target vehiclespeed can be achieved, the driving system is supposed to be controlledthrough running control to eliminate the difference between the bloatedactual driving force or the bloated actual driving torque and therequired driving force or the required driving torque. Such controlconstitutes a factor in disturbances of the behavior of the vehicle orthe occurrence of a sudden rise or a stagnation in vehicle speed.However, according to the foregoing aspect of the invention, in a periodin which the target vehicle speed cannot be achieved, (the absolutevalue of) the required driving force or the required driving torque isrestrained from increasing by limiting the increase or the decrease inthe required driving force or the required driving torque. Thus, theactual driving force or the actual driving torque is restrained fromsuddenly rising or falling upon reestablishment of the state where thetarget vehicle speed can be achieved. Besides, the actual vehicle speedcan reach the target vehicle speed as swiftly as possible when vehiclespeed control by the apparatus in the first aspect of the invention isresumed in response to the reestablishment of the state where the targetvehicle speed can be achieved, by reflecting a tendency of changes inthe difference between the target vehicle speed and the actual vehiclespeed to a certain extent on the required driving force or the requireddriving torque through a method of limiting the increase or the decreasein the required driving force or the required driving torque instead ofstopping running control itself.

It is appropriate for the limitation of the increase or the decrease inthe required driving force or the required driving torque in the periodin which the target vehicle speed according to running control in thisaspect of the invention cannot be achieved to be carried out, as oneaspect of implementation, such that the required driving force or therequired driving torque is limited to prevent the difference between theactual driving force or the actual driving torque that is actuallygenerated in the driving system and the required driving force or therequired driving torque from deviating from “a predetermined limitrange”. According to this configuration, even when a situation in whichthe integrated value of the difference between the target vehicle speedand the actual vehicle speed can increase occurs after continuation ofthe period in which the target vehicle speed cannot be achieved, thevalue of the required driving force or the required driving torque isadjusted such that the difference between the required driving force orthe required driving torque and the actual driving force or the actualdriving torque is confined within the predetermined limit range. Then,the actual driving force or the actual driving torque is restrained fromsuddenly varying upon reestablishment of the state where the targetvehicle speed can be achieved, namely, upon establishment of the statewhere the required driving force or the required driving torque isreflected in the driving system.

In the foregoing aspect of the invention, the width of “thepredetermined limit range” be set such that the required driving forceor the required driving torque can be shifted to a certain extent fromthe actual driving force or the actual driving torque in accordance withthe integrated value of the difference between the target vehicle speedand the actual vehicle speed instead of being set substantially to 0(i.e., instead of making moves of the required driving force or therequired driving torque coincident with those of the actual drivingforce or the actual driving torque). Thus, the required driving force orthe required driving torque at the time of resumption of the vehiclespeed control of this aspect of the invention, which has been suspendedthrough interrupt control, can be made close to a driving force or adriving torque that is originally supposed to be achieved in the vehiclespeed control of this aspect of the invention. When the width of thepredetermined limit range is too small, the actual driving force or theactual driving torque changes slowly at the time of reflection of therequired driving force or the required driving torque, and the follow-upperformance of the actual vehicle speed to the target vehicle speed maydeteriorate. In the meantime, when the width of the predetermined limitrange is too large, the actual driving force or the actual drivingtorque tends to change greatly at the time of reflection of the requireddriving force or the required driving torque. Accordingly, in anembodiment of the invention, it is appropriate for the width of thepredetermined limit range to be experimentally or theoretically decidedin advance such that the actual driving force or the actual drivingtorque does not suddenly rise or fall upon reestablishment of the statewhere the target vehicle speed can be achieved.

It is appropriate for the value of the required driving force or therequired driving torque to be, at the time of reestablishment of thestate where the required driving force or the required driving torque isat least reflected, within a predetermined limit range from the actualdriving force or the actual driving torque at that time. Accordingly, asanother aspect of limiting the increase or the decrease in the requireddriving force or the required driving torque, the required driving forceor the required driving torque at the time when the determination unitdetermines that a change has been made from the state where the requireddriving force or the required driving torque cannot be realized in thedriving system to the state where the required driving force or therequired driving torque can be realized in the driving system may belimited such that the difference between the required driving force orthe required driving torque and the actual driving force or the actualdriving torque of the driving system at the time of the determinationdoes not deviate from the predetermined limit range.

Furthermore, as still another aspect of implementation of the limitationof the increase or the decrease in the required driving force or therequired driving torque in the period in which the target vehicle speedaccording to the aforementioned running control, while the determinationunit determines that the state where the required driving force or therequired driving torque cannot be realized in the driving system isestablished, the value of the required driving force or the requireddriving torque may be held at the required driving force or the requireddriving torque at the time of the determination. In a vehicle, as arule, a driving force or a driving torque that is necessitated toequalize an actual vehicle speed with a target vehicle speed remainssubstantially unchanged as long as there is no substantial change in arunning state of the vehicle. Accordingly, even in the case where thestate where vehicle speed control is reflected is reestablished aftertemporary establishment of the state where vehicle speed control is notreflected, which results from the intervention of additional control,during the functioning of the running control according to this aspectof the invention, it is likely that the driving force or the drivingtorque that is needed for the control of the actual vehicle speed may inmany cases be substantially equal to a value immediately before theintervention of the additional control, as long as there is no change inthe setting of the target vehicle speed. Thus, as described above, it isappropriate to limit the change in the required driving force or therequired driving torque by maintaining the value of the required drivingforce or the required driving torque as soon as it is determined thatthe required driving force or the required driving torque cannot berealized in the driving system, namely, by suspending the integration ofthe difference between the target vehicle speed and the actual vehiclespeed, and to restore a state prior to the intervention of theadditional control by ensuring that the maintained required drivingforce or the maintained required driving torque is reflected in thedriving system when the state where the required driving force or therequired driving torque can be realized in the driving system isreestablished. It is appropriate for the required driving force or thedriving torque at the time of the intervention of the additional controlto be stored and used as the value of the required driving force or therequired driving torque when vehicle speed control is resumed after thetermination of the additional control.

In the first aspect of the invention, in the case where the drivingsystem of the vehicle is an engine, the required driving output decidingunit may include a unit that calculates a target throttle opening degreeof the engine on the basis of a required driving force or a requireddriving torque, a unit that controls controlling a throttle openingdegree of the engine to the target throttle opening degree, and a unitthat detects the throttle opening degree of the engine, and control thethrottle opening degree to output the required driving force or therequired driving torque in the driving system. In this case, it isappropriate to determine whether or not the required driving force orthe required driving torque is unrealizable in the control of thedriving system, by comparing a detected value of the throttle openingdegree with the target throttle opening degree and determining whetheror not a difference between the detected value of the throttle openingdegree and the target throttle opening degree is outside a predeterminedreference range. When a significant difference is detected between thedetected value of the throttle opening degree and the target throttleopening degree (when the difference is beyond the predeterminedreference range), the state where the control according to this aspectof the invention is not reflected is likely to have been established. Inthis case, it is appropriate to determine that the required drivingforce or the required driving torque is unrealizable in the control ofthe driving system. As a matter of course, it is also appropriate todetermine that the intervention of additional control has taken place byacquiring information from the additional control. However, according tothe aforementioned method of determination, even if the controlapparatus is not incorporated with a configuration for acquiringinformation from the additional control, it is possible to determinewhether or not the required driving force or the required driving torqueis unrealizable in the control of the driving system.

The first aspect of the invention curbs an inconvenience resulting froma phenomenon in which the control of the driving force or the drivingtorque for the driving system by the running control apparatus istemporarily suspended due to the intervention of the additional controlduring the actuation of the apparatus. Especially, owing to a progressmade in the art of vehicle running control in recent years, in morecases than ever, a plurality of kinds of control are simultaneouslyperformed in a single vehicle. In the case where the plurality of suchkinds of control are simultaneously performed, the first aspect of theinvention allows these kinds of control to function compatibly with oneanother, and is very useful in a vehicle endowed with a variety ofcontrol functions.

In the foregoing aspect of the invention, it should be noted that, asdescribed already, the running control apparatus is configured such thatthe required driving force or the required driving torque is allowed toshift from the actual driving force or the actual driving torque withinthe predetermined limit range having the width substantially unequal to0 even in the case of the establishment of the state where therequirement made by the apparatus of the invention is not reflected inthe driving system. According to this configuration, at the time whenvehicle speed control is resumed in response to the reestablishment ofthe state where the requirement made by the apparatus of this aspect ofthe invention is reflected in the driving system, the required drivingforce or the required driving torque is offset from the actual drivingforce or the actual driving torque at that time in a directionreflecting the tendency of changes in the difference between the targetvehicle speed and the actual vehicle speed (close to the requireddriving force or the required driving torque that is given on the basisof the integrated value of the difference between the target vehiclespeed and the actual vehicle speed). Thus, good follow-up performance ofthe actual vehicle speed to the target vehicle speed is ensured afterthe resumption of vehicle speed control. Furthermore, according to thisconfiguration, even in the case where the actual driving force or theactual driving torque does not coincide with the required driving forceor the required driving torque due to changes in the condition of a roadsurface, an environment in which the vehicle is used, an ageddeterioration thereof, or the like in the driving system of the vehicle,good vehicle speed control can be resumed. Therefore, an advantage isgained. In the case where a command value for the required driving forceor the required driving torque cannot be accurately or appropriatelygenerated in the driving system, especially where the generated drivingforce or the generated driving torque is smaller than a required valuethereof, if vehicle speed control according to this aspect of theinvention is resumed with the required driving force or the requireddriving torque coincident with the actual driving force or the actualdriving torque, the follow-up performance of the actual vehicle speed tothe target vehicle speed deteriorates after the resumption of vehiclespeed control. However, according to the aforementioned configuration,the required driving force or the required driving torque is offset fromthe actual driving force or the actual driving torque in the directionreflecting the tendency of the changes in the difference between thetarget vehicle speed and the actual vehicle speed. Therefore, the actualvehicle speed can be more swiftly made close to the target vehiclespeed.

In a second aspect of the invention, a running control method for avehicle includes calculating a difference between a target vehicle speedand an actual vehicle speed, deciding a required driving force or arequired driving torque of a driving system of the vehicle on the basisof an integrated value of the difference between the target vehiclespeed and the actual vehicle speed such that the actual vehicle speedcoincides with the target vehicle speed, controlling actuation of thedriving system on the basis of the required driving force or therequired driving torque, determining whether or not the required drivingforce or the required driving torque is unrealizable in the control ofthe driving system due to intervention of additional control, andimposing a limit on an increase or a decrease in the required drivingforce or the required driving torque when it is determined that therequired driving force or the required driving torque is unrealizable inthe control of the driving system.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further objects, features and advantages of theinvention will become apparent from the following description of exampleembodiments with reference to the accompanying drawings, wherein likenumerals are used to represent like elements:

FIG. 1A shows a schematic diagram of an automobile incorporated with arunning control apparatus as an embodiment of the invention. FIG. 1Brepresents, in the form of a control block diagram, a control processingin decision of a required driving torque by the running controlapparatus;

FIGS. 2A to 2D are diagrams exemplifying changes over time in requireddriving torque (dotted lines), actual driving torque (solid line),target vehicle speed (dotted lines), and actual vehicle speed (solidline) in a case where interrupt control intervenes during theperformance of vehicle speed control by the running control apparatus,each showing a case where the actual vehicle speed is lower than thetarget vehicle speed. FIGS. 2A and 2B show changes in driving torque andvehicle speed respectively in a case where the required driving torqueis not limited during the functioning of interrupt control (relatedart). FIGS. 2C and 2D show changes in driving torque and vehicle speedrespectively in a case where the required driving torque is limitedduring the functioning of interrupt control (this embodiment of theinvention);

FIGS. 3A to 3D are diagrams similar to FIGS. 2A to 2D respectively.These diagrams exemplify changes over time in required driving torque,actual driving torque, target vehicle speed, and actual vehicle speed ina case where interrupt control intervenes during the performance ofvehicle speed control by the running control apparatus, each showing acase where the actual vehicle speed is higher than the target vehiclespeed. FIGS. 3A and 3B show changes in driving torque and vehicle speedrespectively in a case where the required driving torque is not limitedduring the functioning of interrupt control (related art). FIGS. 3C and3D show changes in driving torque and vehicle speed respectively in acase where the required driving torque is limited during the functioningof interrupt control (this embodiment of the invention);

FIGS. 4A to 4D are diagrams similar to FIGS. 2A to 2D respectively.These diagrams show examples of changes over time in required drivingtorque, actual driving torque, target vehicle speed, and actual vehiclespeed in a case where interrupt control intervenes during theperformance of vehicle speed control by the running control apparatuswhen the actual driving torque is not suitably generated for therequired driving torque (when the actual driving torque is smaller thanthe required driving torque) in a driving system of a vehicle due to anenvironment in which the vehicle is used, an aged deterioration thereof,or the like. FIGS. 4A and 4B show changes in driving torque and vehiclespeed respectively in a case where the required driving torque is madesubstantially coincident with the actual driving torque during thefunctioning of interrupt control. FIGS. 4C and 4D show changes indriving torque and vehicle speed respectively in a case where therequired driving torque is variable within a predetermined limit rangeduring the functioning of interrupt control; and

FIG. 5 represents, in the form of a flowchart, a control processing ofthe driving control device according to the embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENT

The invention will be described hereinafter in detail with reference tothe accompanying drawings as to an embodiment thereof. In the drawings,like reference symbols represent like parts.

(Configuration of Apparatus) FIG. 1A schematically shows an automobilemounted with a running control apparatus according to the embodiment ofthe invention. In FIG. 1A, a vehicle 10 having a front-left wheel 12FL,a front-right wheel 12FR, a rear-left wheel 12RL, and a rear-right wheel12RR is mounted with a driving system 20 for generating a driving forcefor each of the wheels (only the rear wheels in an example illustratedin FIG. 1A because the vehicle 10 is a rear-wheel-drive vehicle) inaccordance with depression of an accelerator pedal 14 by a driver in anormal mode. In the illustrated example, the driving system 20 isdesigned such that a driving torque or a rotational driving force thatis output from an engine 22 via a torque converter 24, an automatictransmission 26, a differential gear mechanism 28, and the like istransmitted to each of the rear wheels 12RL and 12RR. The driving system20 may be an electric driving system employing a motor instead of theengine 22, or a hybrid driving system having both an engine and a motor.Although not illustrated for the sake of simplicity, as is the case withnormal vehicles, the vehicle 10 is provided with a braking device forgenerating a braking force for each of the wheels, and a steering gearfor controlling the steering angle of the front wheels or the steeringangles of the front wheels and the rear wheels. The braking device has ahydraulic circuit including an oil reservoir, an oil pump, variousvalves and the like, wheels cylinders fitted on the respective wheels,and a master cylinder that is actuated in response to depression of abrake pedal by the driver. Brake pressures in the respective wheelcylinders, namely, braking forces applied to the respective wheels areadjusted by the hydraulic circuit in response to a pressure in themaster cylinder. However, the braking device may be designed toelectromagnetically apply a braking force to each of the wheels, or bedesigned in other arbitrary forms known to those skilled in the art. Thevehicle may be a front-wheel-drive vehicle or a four-wheel-drivevehicle.

The actuation of the driving system 20 is controlled by an electroniccontrol unit 50. Respective means in the running control apparatusaccording to this embodiment of the invention are realized in theconfiguration and actuation of this electronic control unit 50. It isappropriate for the control performed by the running control apparatusaccording to this embodiment of the invention to be an arbitrary type ofautomatic running control for automatically making an actual vehiclespeed of the vehicle coincident with a target vehicle speed, morespecifically, such as cruise control, idle speed control, creep runningcontrol, downhill assist control, or the like. In the case where thedriving system is controlled through additional interrupt controldifferent from running control in this embodiment of the invention, forexample, stall control, traction control, VSC, or the like, any one ofthese types of control is performed in preference to the running controlaccording to this embodiment of the invention. The interrupt control mayalso be realized inside the electronic control unit 50. Alternatively,it is appropriate for the interrupt control to be performed in anotherelectronic control device unit (not shown).

It is appropriate for the electronic control unit 50 to include adriving circuit and a normal-type microcomputer having a CPU, a ROM, aRAM, and an input/output port device that are coupled to one another bya bidirectional common bus. Detected values of a rotational speed Er ofthe engine, a throttle opening degree θs of a throttle valve 22 a of theengine, an accelerator pedal depression amount θa, and the like areinput to the electronic control unit 50 from sensors provided onrespective portions of the vehicle. Detected values of wheel speeds Vwiare input to the electronic control unit 50 from wheel speed sensors 30i (i=FL, FR, RL, RR). In addition to the aforementioned detected values,it is appropriate for various detection signals for obtaining variousparameters needed for various types of control to be performed in thevehicle of this embodiment of the invention to be input. As will bedescribed below in detail, the electronic control unit 50 adjusts theopening degree of the throttle 22 a of the engine 22 on the basis of aseries of aforementioned pieces of information such that the requireddriving torque is achieved when the actual vehicle speed of the vehiclecoincides with the target vehicle speed. It is appropriate for thetarget vehicle speed to be set by a speed setter 52 installed at aposition accessible by the driver of the vehicle (e.g., on a steeringwheel, a front panel, or the like).

FIG. 1B shows, in the form of a control block, the configuration of acontrol processing for deciding a required driving torque from a targetvehicle speed and an actual vehicle speed in the electronic control unit50 for vehicle speed control. It should be understood in FIG. 1B that adriving output command value of the driving system is calculated inunits of driving torque but may also be calculated in units of drivingforce generated in each of the driving wheels of the vehicle.

Referring to FIG. 1B, the control configuration according to thisembodiment of the invention includes, as is the case with theconfiguration of normal vehicle speed control and as shown in FIG. 1B,the configuration of feedback control in which a difference ΔV=Vt−Vabetween a target vehicle speed Vt given by the speed setter 52 and anactual vehicle speed Va estimated from wheel speeds or the like iscalculated (an adder a1, vehicle speed difference calculating means),the difference ΔV is serially integrated to eliminate the difference ΔV,and a required driving torque Tt to be output in the driving system iscalculated. It is appropriate for the required driving torque Tt to begiven according to, for example, Tt=ΔT+Tt (last value) . . . (1) (anadder a2). It should be noted herein that ΔT is a torque change amountobtained using a map (not shown) having the current vehicle speeddifference ΔV as a variable. Typically, this torque change amount is aquantity given in the form of ΔT=K₁·ΔV . . . (2) (a multiplier ml). K₁is a feedback gain. This feedback gain may be a constant, but may begiven by a map having the vehicle speed difference ΔV as a variable.Further, Tt (last value) is a required driving torque already obtainedup to now, namely, an integrated value of the ΔT integrated so far.Thus, according to the required driving torque in an expression (1), inthe case where the actual vehicle speed Vt is different from the targetvehicle speed Va when the target vehicle speed Va is given as a certainvalue, a change in Tt is caused, and the vehicle isaccelerated/decelerated until the actual vehicle speed Vt coincides withthe target vehicle speed Va. Then, when the actual vehicle speed Vtcoincides with the target vehicle speed Va, the driving torque resultingfrom the integrated value of ΔT corresponding to the vehicle speeddifference ΔV obtained so far counterbalances a force foraccelerating/decelerating the vehicle, for example, a gradient of a roadsurface or a running resistance to maintain the vehicle speed.

The required driving torque Tt thus calculated is then used to calculatea target throttle opening degree Ost of the driving system. However, aswill be described later, in the case where interrupt control performedin preference to the control by the apparatus according to thisembodiment of the invention intervenes in controlling the engine, therequired driving torque Tt is not directly used as a control commandvalue for the engine. Instead, the required driving torque is subjectedto an upper/lower limit guard G1 that is set by referring to an actualdriving torque Ta actually generated in the engine. Hence, the value ofthe required driving torque Tt is limited. Even in the case where thecontrol according to this embodiment of the invention is realized in thedriving system, it is appropriate to perform a processing such as anupper/lower limit guard for limiting a maximum value or a minimum valueof the required driving torque Tt, a change amount guard for limiting amaximum value or a minimum value of the change in the required drivingtorque Tt, or the like, for the sake of operational safety or protectionof the driving system or the vehicle.

In the upper/lower limit guard G1 for limiting the value of the requireddriving torque that is used to calculate the target throttle openingdegree Ost of the driving system during the performance of interruptcontrol, the value of the required driving torque is limited to a rangedefined by Ta−Tlg≦Tt≦Ta+Tug . . . (3). It should be noted herein thatTlg and Tug are a lower limit and an upper limit of the required drivingtorque respectively, which are measured from the actual driving torqueTa. That is, during the performance of interrupt control, the requireddriving torque Tt is allowed to be variable only within a predeterminedlimit range having the actual driving torque Ta as a criterion. In thiscase, as understood from FIG. 1B, the integration of ΔT with therequired driving torque in the expression (1) is also carried out usingthe required driving torque limited to the range of an expression (3) asTt (last value). Thus, as long as the value of the required drivingtorque calculated in the expression (1) is limited to the range of theexpression (3) measured from the actual driving torque, the value of thecurrent vehicle speed difference ΔV is reflected. Even in the case wherethe value of the required driving torque in the expression (1) exceedsthe value in the expression (3), the required driving torque is set toan upper limit or a lower limit that corresponds to the sign of theintegrated value of the vehicle speed difference ΔV (i.e., theintegrated value of ΔT). Then, after the termination of interruptcontrol, vehicle speed control according to this embodiment of theinvention is resumed using the required driving torque that reflects, toa certain extent, a tendency of the vehicle speed difference ΔVdemonstrated so far.

Although not shown, a feedforward control portion is usually provided inaddition to the aforementioned configuration of feedback control. In thefeedforward control portion, a feedforward component T_(FF) of therequired driving torque is calculated. In the case where, for example,the road surface on which the vehicle runs has a gradient, thefeedforward component T_(FF) of the required driving torqueincreases/reduces the required driving torque in accordance with thegradient of the road surface. In the case where this feedforward controlis used as well, with the required driving torque resulting fromfeedback control that is given as the integrated value of ΔV (theintegrated value of ΔT) expressed as a feedback component T_(FB), atotal required driving torque Tt is given according to Tt=T_(FB)+T_(FF). . . (4). In integrating the vehicle speed difference ΔV, thefeedforward component T_(FF) needs to be removed. In consequence,therefore, the required driving torque is given according to Tt=ΔT+Tt(last value)−T_(FF) (last value)+T_(FF) . . . (5). During theperformance of interrupt control, Tt is limited to the range of theexpression (3) by the same token. A value limited to the range of theexpression (3) is used as Tt (last value).

(Actuation of Apparatus) FIGS. 2A to 4D show examples of changes overtime in required driving torque (dotted lines), actual driving torque(solid line), target vehicle speed (dotted lines), and actual vehiclespeed (solid line) in a case where additional interrupt controlintervenes during the actuation of the running control apparatusaccording to this embodiment of the invention. For comparison, anexemplary case of the running control apparatus in the related art isalso illustrated.

Reference will be made first to FIGS. 2A to 2D. In the case where therequired driving torque is not limited during the functioning ofinterrupt control (related art) as shown in FIGS. 2A and 2B when theactual vehicle speed is lower than the target vehicle speed due tointerrupt control, the actual driving torque suddenly increases (FIG.2A) after the resumption of vehicle speed control, and the actualvehicle speed becomes much higher than the target vehicle speed inresponse thereto (FIG. 2B). On the other hand, in the case where therequired driving torque is limited during the functioning of interruptcontrol (this embodiment of the invention) as shown in FIGS. 2C and 2D,the required driving torque during the functioning of interrupt controlis allowed to change to a value larger than the actual driving torque byTug (FIG. 2C). In this case, the actual vehicle speed swiftly followsthe target vehicle speed after the resumption of vehicle speed control(FIG. 2D).

Reference will be made next to FIGS. 3A to 3D. In the case where therequired driving torque is not limited during the functioning ofinterrupt control (related art) as shown in FIGS. 3A and 3B when theactual vehicle speed is higher than the target vehicle speed due tointerrupt control, the required driving torque drastically decreasesduring the functioning of interrupt control. Therefore, the actualdriving torque is substantially 0 until the required driving torquebecomes positive after the resumption of vehicle speed control. As aresult, a stagnation in vehicle speed is caused. On the other hand, inthe case where the required driving torque is limited during thefunctioning of interrupt control (this embodiment of the invention) asshown in FIGS. 3C and 3D, the required driving torque during thefunctioning of interrupt control is allowed to change to a value smallerthan the actual driving torque by Tlg (FIG. 3C). In this case, theactual vehicle speed swiftly follows the target vehicle speed after theresumption of vehicle speed control (FIG. 3D).

As described above, even in the case where the running control apparatusin the related art, namely, additional interrupt control intervenes,when the required driving torque Tt is not limited according to theexpression (3), the actual vehicle speed Va diverges from the targetvehicle speed Vt to continue a state of ΔV≠0 during the functioning ofinterrupt control as shown in FIG. 2B or FIG. 3B, and the requireddriving torque Tt continues to rise (FIG. 2A) or fall (FIG. 3A). Afterthat, when vehicle speed control by the running control apparatus isresumed in response to the termination of interrupt control, the actualdriving torque Ta suddenly increases (FIG. 2A) or falls (FIG. 3A) toachieve the required driving torque Tt at that time. In consequence, thevehicle speed increases (FIG. 2B) or stagnates (FIG. 3B).

On the other hand, as described above, in the case where the requireddriving torque Tt is limited to the range of the expression (3) by theupper/lower limit guard during the functioning of interrupt control,even when ΔT is added in the expression (1) or (5), the required drivingtorque Tt changes substantially along the actual driving torque Ta, asshown in FIG. 2C or FIG. 3C. Accordingly, when vehicle speed control bythe running control apparatus is resumed in response to the terminationof interrupt control, sudden variations in the actual driving torque aresuppressed. As a result, the actual vehicle speed swiftly follows thetarget vehicle speed (FIG. 2D or FIG. 3D).

As described already, when the required driving torque Tt is setvariable in the direction in which the integrated value of ΔT isreflected instead of being made completely coincident with the actualdriving torque Ta during the functioning of interrupt control, theactual vehicle speed can be made to more swiftly follow the targetvehicle speed even in the case where the required driving torque Ttcannot be generated in the driving system due to an environment in whichthe vehicle is used, an aged deterioration thereof, or the like.

FIGS. 4A to 4D show the aforementioned circumstances. FIGS. 4A and 4Bshow changes in driving torque and vehicle speed respectively in thecase where the required driving torque is made substantially coincidentwith the actual driving torque during the functioning of interruptcontrol. FIGS. 4C and 4D show changes in driving torque and vehiclespeed respectively in the case where the required driving torque isvariable within a predetermined limit range during the functioning ofinterrupt control. As understood from FIGS. 4A to 4D, when vehicle speedcontrol is resumed, the required driving torque starts rising from theactual driving torque at that time in accordance with the vehicle speeddifference in the case of FIG. 4A, but rises from a value larger thanthe actual driving torque at that time by Tug in accordance with thevehicle speed difference in the case of FIG. 4C. That is, in the casewhere the required driving torque Tt is made completely coincident withthe actual driving torque Ta during the functioning of interruptcontrol, when vehicle speed control by the running control apparatus isresumed, variations in the required driving torque occur after theintegration of ΔT has been awaited. Accordingly, changes in vehiclespeed are delayed (FIGS. 4A and 4B). Meanwhile, if the required drivingtorque Tt is shifted from the actual driving torque Ta in the directionin which the integrated value of ΔT is reflected, vehicle speed controlis resumed with a larger required driving torque at the time of thetermination of interrupt control (FIGS. 4C and 4D).

FIG. 5 represents, as a flowchart, the flow of a processing process inthe electronic control unit 50 for realizing running control accordingto this embodiment of the invention. The control processing of FIG. 5 isrepeatedly performed at intervals in the order of millisecond whenautomatic control of vehicle speed is requested through an input made bythe driver (e.g., by turning an actuating switch ON).

Referring to FIG. 5, when the cycle of control is started, the currentactual vehicle speed Va is first detected or estimated. It isappropriate for the actual vehicle speed Va to be decided on the basisof data on the wheel speeds Vwi from the wheel speed sensors provided onthe respective wheels according to an arbitrary method (e.g., as anaverage of the wheel speeds of all the wheels, the driving wheels, ordriven wheels or the like) (step 10). After that, the required drivingtorque Tt is calculated according to the expressions (1) and (2) or theexpressions (5) and (2) (step 20).

It is then determined whether or not interrupt control, which isperformed in preference to vehicle speed control as running controlaccording to this embodiment of the invention, is functioning, namely,whether or not a requirement for vehicle speed control is reflected(step 30). This determination may be made by receiving a signalindicating information on whether or not the control of the drivingsystem is performed by an interrupt control. However, as will bedescribed later, this determination may be made by referring to thestate of the throttle opening degree. When it is determined in thedetermination of step 30 that the requirement for vehicle speed controlis reflected, the required driving torque Tt calculated in step 20 isconverted into a target throttle opening degree according to anarbitrary method known to those skilled in the art (step 40). The targetthrottle opening degree is transmitted to a control device (a throttleopening degree actuator (not shown)) of the driving system or the like(step 50).

On the other hand, when it is determined in the determination of step 30that the requirement for vehicle speed control is not reflected, thevalue of the required driving torque Tt is limited to the range definedby the aforementioned expression (3). It is appropriate for thisprocessing to be performed through, for example, a substitutionprocessing of (step 60) Tt←MAX (Tt, Ta−Tlg) . . . (6a) (step 70) Tt←MIN(Tt, Ta+Tug) . . . (6b) (it should be noted herein that MAX representsthe selection of a maximum value and that MIN represents the selectionof a minimum value). It is appropriate for the actual driving torque Tagenerated in the engine to be estimated using, for example, a map havinga current throttle opening degree, an engine rotational speed, or thelike as a variable. It is appropriate for the upper limit Tug and thelower limit Tlg to be experimentally or theoretically decided such thata sudden rise or fall in the actual driving force or the actual drivingtorque does not occur in resuming vehicle speed control.

Thus, when the required driving torque Tt, which is restrained fromvarying, is calculated, a conversion of the required driving torque Ttinto the target throttle opening degree is made (step 40), and thetarget throttle opening degree is transmitted to the control device ofthe driving system as a command value (step 50). As described already,the required driving torque Tt, which has undergone step 60 and step 70,is utilized as a last value in step 20 of a subsequent control cycle. Asunderstood from the foregoing description, when the requirement for thecontrol of the driving system by the running control apparatus of theinvention is reflected after the termination of interrupt control, therequired driving torque, which has undergone step 60 and step 70 in aprevious cycle and has been obtained by adding ΔT to the requireddriving torque as a value in the neighborhood of the actual drivingtorque Ta, is output to the driving system. As a result, suddenvariations in torque are avoided in the driving system.

(Determination on Requirement Reflecting State) As described already, itis appropriate for the determination on whether or not the requirementfor vehicle speed control is reflected in step 30 of the flowchart ofFIG. 5 to be made depending on whether or not the detected value θs ofthe throttle opening degree substantially coincides with the targetthrottle opening degree θst calculated in step 40 of the flowchart ofFIG. 5. For example, when there is established a relationship|θs−θst|>Δθs . . . (7) (it should be noted herein that Δθs is apositive, very small constant) as to the absolute value of thedifference between the detected value of the throttle opening degree andthe target throttle opening degree, there is a significant differencebetween the detected value θs of the throttle opening degree and thetarget throttle opening degree θst, and the control of the drivingsystem for vehicle speed control according to this embodiment of theinvention is not reflected owing to interrupt control. In other words,it is appropriate to determine that there is not established a state of“requirement reflected”. According to this method, it is possible tofind out whether or not running control according to this embodiment ofthe invention is reflected, without using a configuration for acquiringfrom interrupt control information on the state of the functioningthereof.

The invention has been described above in detail as to the specificembodiment thereof. However, it would be obvious to those skilled in theart that the invention is not limited to the foregoing embodimentthereof and that a variety of other embodiments are possible within thescope of the invention.

For example, in respect of the limitation of the value of the requireddriving torque Tt during the functioning of interrupt control, it isappropriate for the required driving torque Tt to be limited, asdescribed already. For example, the execution of step 60 and step 70 maybe allowed only in a cycle in which it is determined in step 30 of FIG.5 that temporarily functioning interrupt control has been terminated.During interrupt control, the required driving torque Tt is notreflected and hence may be an arbitrary value. For example, from thebeginning of the functioning of interrupt control to the terminationthereof, the addition of ΔT according to the expression (1) or (5) maybe suspended to thereby maintain the value of the required drivingtorque. In this case, after the termination of the functioning ofinterrupt control, the control of the driving system is resumed in astate prior to the beginning thereof.

Although vehicle speed control through the control of the driving systemis described in the description of the foregoing embodiment of theinvention, it should be understood that the control apparatus may beautomatically actuated in an appropriate manner when the vehicle speedexceeds the target vehicle speed.

1. A running control apparatus for a vehicle, comprising: a vehiclespeed difference calculating unit that calculates a difference between atarget vehicle speed and an actual vehicle speed; a required drivingoutput deciding unit that decides a required driving force or a requireddriving torque of a driving system of the vehicle on a basis of anintegrated value of the difference between the target vehicle speed andthe actual vehicle speed such that the actual vehicle speed coincideswith the target vehicle speed; a driving control unit that controlsactuation of the driving system on a basis of the required driving forceor the required driving torque; and a determination unit that determineswhether or not the required driving force or the required driving torqueis unrealizable in the control of the driving system due to interventionof additional control, wherein the required driving output deciding unitimposes a limit on an increase or a decrease in the required drivingforce or the required driving torque when the determination unitdetermines that the required driving force or the required drivingtorque is unrealizable in the control of the driving system, wherein avalue of the required driving force or the required driving torque ismaintained, while the determination unit determines that the requireddriving force or the required driving torque is unrealizable in thecontrol of the driving system, at a value of the required driving forceor the required driving torque at a time when it is determined that therequired driving force or the required driving torque is unrealizable inthe control of the driving system.
 2. The running control apparatusaccording to claim 1, wherein the required driving force or the requireddriving torque is limited such that a value of the required drivingforce or the required driving torque becomes closer to an actual drivingforce or an actual driving torque of the driving system than to a valueof the required driving force or the required driving torque that isdecided on a basis of an integrated value of the difference between thetarget vehicle speed and the actual vehicle speed, in a case where therequired driving output deciding unit imposes the limit on the increaseor the decrease in the required driving force or the required drivingtorque when it is determined that the required driving force or therequired driving torque is unrealizable in the control of the drivingsystem.
 3. The running control apparatus according to claim 1, whereinthe driving system of the vehicle is an engine, the required drivingoutput deciding unit includes a unit that calculates a target throttleopening degree of the engine on a basis of the required driving force orthe required driving torque, a unit that controls a throttle openingdegree of the engine to the target throttle opening degree, and a unitthat detects the throttle opening degree of the engine, and thedetermination unit determines that the required driving force or therequired driving torque is unrealizable in the control of the drivingsystem, when a difference between a detected value of the throttleopening degree and the target throttle opening degree is outside apredetermined reference range.
 4. The running control apparatusaccording to claim 1, wherein the target vehicle speed is set by adriver of the vehicle.
 5. The running control apparatus according toclaim 1, wherein the target vehicle speed is set on a basis of a drivemaneuver amount of a driver of the vehicle.
 6. The running controlapparatus according to claim 1, wherein the target vehicle speed is seton a basis of a gradient of a road surface on which the vehicle runs. 7.A running control apparatus for a vehicle, comprising: a vehicle speeddifference calculating unit that calculates a difference between atarget vehicle speed and an actual vehicle speed; a required drivingoutput deciding unit that decides a required driving force or a requireddriving torque of a driving system of the vehicle on a basis of anintegrated value of the difference between the target vehicle speed andthe actual vehicle speed such that the actual vehicle speed coincideswith the target vehicle speed; a driving control unit that controlsactuation of the driving system on a basis of the required driving forceor the required driving torque; and a determination unit that determineswhether or not the required driving force or the required driving torqueis unrealizable in the control of the driving system due to interventionof additional control, wherein the required driving output deciding unitimposes a limit on an increase or a decrease in the required drivingforce or the required driving torque when the determination unitdetermines that the required driving force or the required drivingtorque is unrealizable in the control of the driving system, wherein avalue of the required driving force or the required driving torque islimited such that a difference between the required driving force or therequired driving torque and an actual driving force or an actual drivingtorque of the driving system at a time, when it is determined that astate of the required driving force or the required driving torquechanges from a state that the required driving force or the requireddriving torque is unrealizable in the control of the driving system intoa state that the required driving force or the required driving torqueis realizable in the driving system, does not deviate from apredetermined limit range.
 8. The running control apparatus according toclaim 7, wherein the required driving force or the required drivingtorque is limited such that a value of the required driving force or therequired driving torque becomes closer to an actual driving force or anactual driving torque of the driving system than to a value of therequired driving force or the required driving torque that is decided ona basis of an integrated value of the difference between the targetvehicle speed and the actual vehicle speed, in a case where the requireddriving output deciding unit imposes the limit on the increase or thedecrease in the required driving force or the required driving torquewhen it is determined that the required driving force or the requireddriving torque is unrealizable in the control of the driving system. 9.The running control apparatus according to claim 7, wherein the drivingsystem of the vehicle is an engine, the required driving output decidingunit includes a unit that calculates a target throttle opening degree ofthe engine on a basis of the required driving force or the requireddriving torque, a unit that controls a throttle opening degree of theengine to the target throttle opening degree, and a unit that detectsthe throttle opening degree of the engine, and the determination unitdetermines that the required driving force or the required drivingtorque is unrealizable in the control of the driving system, when adifference between a detected value of the throttle opening degree andthe target throttle opening degree is outside a predetermined referencerange.
 10. The running control apparatus according to claim 7, whereinthe target vehicle speed is set by a driver of the vehicle.
 11. Therunning control apparatus according to claim 7, wherein the targetvehicle speed is set on a basis of a drive maneuver amount of a driverof the vehicle.
 12. The running control apparatus according to claim 7,wherein the target vehicle speed is set on a basis of a gradient of aroad surface on which the vehicle runs.
 13. A running control method fora vehicle, comprising: calculating a difference between a target vehiclespeed and an actual vehicle speed; deciding a required driving force ora required driving torque of a driving system of the vehicle on a basisof an integrated value of the difference between the target vehiclespeed and the actual vehicle speed such that the actual vehicle speedcoincides with the target vehicle speed; controlling actuation of thedriving system on a basis of the required driving force or the requireddriving torque; determining whether or not the required driving force orthe required driving torque is unrealizable in the control of thedriving system due to intervention of additional control; and imposing alimit on an increase or a decrease in the required driving force or therequired driving torque when it is determined that the required drivingforce or the required driving torque is unrealizable in the control ofthe driving system, wherein the required driving force or the requireddriving torque is maintained, while it is determined that the requireddriving force or the required driving torque is unrealizable in thecontrol of the driving system, at a value of the required driving forceor the required driving torque at a time when it is determined that therequired driving force or the required driving torque is unrealizable inthe control of the driving system.
 14. The running control methodaccording to claim 13, wherein the required driving force or therequired driving torque is limited such that a value of the requireddriving force or the required driving torque becomes closer to an actualdriving force or an actual driving torque of the driving system than toa value of the required driving force or the required driving torquethat is decided on a basis of an integrated value of the differencebetween the target vehicle speed and the actual vehicle speed, inimposing the limit on the increase or the decrease in the requireddriving force or the required driving torque when it is determined thatthe required driving force or the required driving torque isunrealizable in the control of the driving system.
 15. The runningcontrol method according to claim 13, wherein the driving system of thevehicle is an engine, the method further comprising: calculating atarget throttle opening degree of the engine on a basis of the requireddriving force or the required driving torque; controlling a throttleopening degree of the engine to the target throttle opening degree,detecting the throttle opening degree of the engine; and determiningthat the required driving force or the required driving torque isunrealizable in the control of the driving system, when a differencebetween a detected value of the throttle opening degree and the targetthrottle opening degree is outside a predetermined reference range. 16.A running control method for a vehicle, comprising: calculating adifference between a target vehicle speed and an actual vehicle speed;deciding a required driving force or a required driving torque of adriving system of the vehicle on a basis of an integrated value of thedifference between the target vehicle speed and the actual vehicle speedsuch that the actual vehicle speed coincides with the target vehiclespeed; controlling actuation of the driving system on a basis of therequired driving force or the required driving torque; determiningwhether or not the required driving force or the required driving torqueis unrealizable in the control of the driving system due to interventionof additional control; and imposing a limit on an increase or a decreasein the required driving force or the required driving torque when it isdetermined that the required driving force or the required drivingtorque is unrealizable in the control of the driving system, wherein avalue of the required driving force or the required driving torque islimited such that a difference between the required driving force or therequired driving torque and an actual driving force or an actual drivingtorque of the driving system at a time, when it is determined that astate of the required driving force or the required driving torquechanges from a state that the required driving force or the requireddriving torque is unrealizable in the control of the driving system intoa state that the required driving force or the required driving torqueis realizable in the driving system, does not deviate from apredetermined limit range.
 17. The running control method according toclaim 16, wherein the required driving force or the required drivingtorque is limited such that a value of the required driving force or therequired driving torque becomes closer to an actual driving force or anactual driving torque of the driving system than to a value of therequired driving force or the required driving torque that is decided ona basis of an integrated value of the difference between the targetvehicle speed and the actual vehicle speed, in imposing the limit on theincrease or the decrease in the required driving force or the requireddriving torque when it is determined that the required driving force orthe required driving torque is unrealizable in the control of thedriving system.
 18. The running control method according to claim 16,wherein the driving system of the vehicle is an engine, the methodfurther comprising: calculating a target throttle opening degree of theengine on a basis of the required driving force or the required drivingtorque; controlling a throttle opening degree of the engine to thetarget throttle opening degree, detecting the throttle opening degree ofthe engine; and determining that the required driving force or therequired driving torque is unrealizable in the control of the drivingsystem, when a difference between a detected value of the throttleopening degree and the target throttle opening degree is outside apredetermined reference range.